1. Field of the Invention
The present invention relates to a microwave device for amplifying low noise, which is used in a receiver for, e.g., a direct broadcast satellite (DBS) system.
2. Related Background Art
Conventionally, a microwave device of this type often employs a microstrip line prepared by forming a metal thin film on a dielectric member. FIG. 1 shows a general structure of the microstrip line. As shown in FIG. 1, a conductive layer 31 is formed on a rear surface of a dielectric member 32 having a thickness 41, and a strip conductor 33 having a width 42 is formed on the front surface of the dielectric member 32, thus constituting a microstrip line.
In the microwave device, a demand has arisen for decreasing the thickness of the dielectric member 32. When the thickness of the dielectric member 32 is decreased, the following advantages are obtained.
First, since the width 42 of the strip conductor 33 can be decreased, chip size can be reduced. Since the characteristic impedance of the microstrip line is expressed by a ratio of the width 42 of the strip conductor 33 to the thickness 41 of the dielectric member 32, if the thickness of the dielectric member 32 is decreased, the width of the strip conductor 33 can also be decreased within a range wherein the ratio is left unchanged.
Second, when the thickness of the dielectric member 32 is decreased, a through hole or "via-hole" connecting the conductive layer 31 and the strip conductor 33 can be rendered shallow, and a transmission loss between the layer 31 and the conductor 33 can be reduced. Thus, low-noise characteristics can be improved.
Third, variations in shape and dimensions of the via-hole can be reduced, and variations in performance of the microwave device can be eliminated.
In this manner, it is important to decrease the thickness of the dielectric member 32 in view of an improvement of the perforance of the microwave device. In particular, since a RF amplifier of a down converter is required to have good low-noise characteristics, if the thickness of the dielectric member can be decreased, a remarkable improvement of the performance can be expected.
However, when the thickness is decreased, the following new problems are posed.
First, if the thickness is excessively decreased in a process of decreasing the thickness of the dielectric member 32, the yield is decreased.
Second, since it is difficult to handle a semiconductor having a decreased thickness, the yield in the process after the decrease in thickness is decreased.
Third, a transmission loss is increased.
As described above, when the thickness of the dielectric member 32 can be decreased, the performance can be improved. However, the thickness of the dielectric member 32 cannot be decreased drastically due to the above-mentioned problems.